1. Field of the Invention
The present invention relates to a method and an apparatus for non-contact three-dimensional surface measurement, which projects a grating pattern onto an object being measured while the phase of the pattern is being shifted, so that the projected pattern is observed in a different direction from that of the projection to analyze the contrast of a grating image deformed in accordance with the shape of the object and thereby obtain the shape thereof. More particularly, the invention relates to a method and apparatus for non-contact three-dimensional surface measurement which is preferably used for providing digitized data on design features to a CAD system as well as for a non-contact digitizer for evaluating the accuracy in shape of prototype or mass-produced parts (in the field of Reverse Engineering), which enables measurement of three-dimensional shapes such as of a full-scale car over a large measurement range at high speeds in a non-contact manner.
2. Description of the Related Art
For example, techniques for high-speed non-contact measurement of three-dimensional shapes are disclosed in Japanese Patent Laid-Open Publication No. Hei 10-246612 (Patent Document 1), U.S. Pat. No. 5,175,601 (Patent Document 2), and U.S. Pat. No. 5,319,445 (Patent Document 3). These documents describe moiré methods employing the moiré topography, in which a grating pattern is projected onto an object being measured to measure the three-dimensional shape of the object from a grating image deformed in accordance with the height distribution of each portion of the object. These moiré methods are divided into two types: the projection moiré and the shadow moiré. As shown in FIG. 1, the projection moiré method employs an optical configuration in which with two gratings G1, G2 for respective projection and observation uses being disposed each before a projection lens L1 and an imaging lens L2, the grating G1 is projected onto an object being measured through the lens L1 so that the grating lines deformed in accordance with the shape of the object are focused on the other grating G2 through the lens L2, thereby producing moiré contour lines at predetermined distances h1, h2, h3 from a reference surface. On the other hand, as shown in FIG. 2, the shadow moiré employs an optical configuration in which with one large grating G being disposed on a reference surface, a point light source S at the projection lens L1, and an observing eye “e” at the imaging lens L2, the shadow of the grating G formed by the light source S is projected onto the object being measured to form the shadow of the grating G deformed in accordance with the shape of the object, so that the shadow is observed with the observing eye “e” through the grating G, thus allowing the moiré fringes produced by the grating G and the deformed grating shadow to be observed. In addition, Japanese Patent Laid-Open Publication No. 2002-267429 (Patent Document 4) describes that a phase shift method can be applied to the shadow moiré method.
Furthermore, a non-contact surface measurement method which combines the projection of a grating pattern with a phase shift as shown in FIG. 3 is described in “Surface dimensional measurement using the grating pattern projection method by detecting phase and contrast” (Non-patent Document 1), by Masahito Tonooka et al, Precision Engineering Transaction of JSPE, Vol. 66, No. 1, pp 132-136 (January, 2000).
This method takes the following procedures to measure the shape of an object.
(1) For example, an illumination lamp 10 illuminates a grating filter 12 disposed before a projection lens 14, thereby allowing a grating pattern to be projected onto a work being measured 8 from a position different from the focal point of an imaging optical system (an imaging lens 20).
(2) A grating shift mechanism 16 moves (phase shifts) the grating filter 12 in the horizontal direction as shown by arrow “A”, such that an image is captured by the pixels of an imaging device 22 via the imaging lens 20, thereby allowing the variations in intensity of the image at the pixels to be converted into sinusoidal waveforms.
(3) Several images are collected each at equal phase shift intervals.
(4) The phase and contrast at each pixel are calculated.
(5) The work 8 is moved stepwise in the direction of height (or in the direction of the focal point or the like) to repeat steps (2) through (4). The work 8 is moved at least twice.
(6) The focal point at which each pixel provides the maximum contrast is determined, and the fringe order is also determined.
(7) The phase at which each pixel provides the maximum contrast is selected to determine a phase distribution.
(8) The difference between the phase and a reference phase is calculated.
(9) The distance in the direction of depth (height) is calculated using the phase difference and the fringe order.
However, according to Patent Documents 1 to 4, the same output is repeatedly delivered each time the phase shifts by 2π as shown in FIG. 4. Accordingly, only the phase shift is not enough to identify the sequential number of a captured line of the projected grating, so that the order of the fringe cannot be determined. Therefore, the measurement range is limited to either one of A, B, C, D, . . . . A measurement range within the order of one fringe would cause the grating spacing to be widened, resulting in degradation in measurement accuracy. On the other hand, the grating spacing may be reduced to ensure high measurement accuracy; however, in this case, the measurement range is reduced in the direction of depth. Additionally, when compared with a pixel in focus, a pixel out of focus is measured with insufficient accuracy.
On the other hand, according to Non-patent Document 1, a less number of measurement steps would produce an error in fitting the Gaussian function to determine the maximum contrast, thereby causing a point to be located with reduced accuracy between steps. Moving times could be increased to reduce the error, but with an increase in measurement time. This would also make the system complicated because of the movements of the work or the illuminating and the imaging optical systems. Furthermore, the measurement range in the direction of depth can be increased by increasing the number of steps; however, constraints were placed on the measurement time and the system itself.